This invention relates, in general, to a method of sensing an object and, more particularly, to a method of sensing a sheet.
Some printing steps include feeding a sheet into a printer, forming characters on the sheet with ink, drying the ink, and expelling the sheet from the printer. An important step in printing with, for example, an inkjet printer is sensing or detecting a position of a sheet in the inkjet printer. Many techniques are known for accomplishing positional detection, including a reflective optical sensing technique wherein a light emitting source and an optical sensor, such as a photodetector, are mounted on the printer. The light emitting source is positioned to transmit a light signal to a known position along a path traversed by the sheet. A portion of the light signal is reflected to the optical sensor. When a sheet arrives at the known position along the path, a change in the amount of light reflected to and received by the photodetector occurs. This change in reflected light indicates the presence of a sheet at the known position, thereby detecting or sensing the sheet.
In an optical transmissive technique, a light emitting source and an optical detector are mounted on the printer such that the light emitting source and the optical detector are on opposite sides of the path traversed by the sheet. In the absence of a sheet, the light emitting source transmits an optical signal to the optical detector. When a sheet is positioned between the light emitting source and the optical detector, the amount of light received by the optical detector changes, thereby detecting the presence of a sheet. Thus, this technique employs optical transmission rather than optical reflection.
Although these techniques offer a means for detecting a sheet, they are not without disadvantages. For example, noise generated by the heater for drying the ink may interfere with the signal from the light emitting source in both the optical reflective and optical transmissive techniques when the light emitting source emits light in the infrared spectrum. Further, the optical transmissive technique is unable to detect a sheet transparent to visible light when the light emitting source emits light in the visible spectrum. In addition, both techniques may include extra circuitry that adds to the complexity and price of the systems.
Accordingly, it would be advantageous to have a method for detecting sheets that is immune to noise sources within the printer and is capable of detecting sheets that are either opaque or transparent to visible light. Moreover, it would be advantageous for the method to decrease the number of components required for detecting sheets, thereby lowering the price of the printer as well as the potential maintenance costs of the printer.